Automatic frequency control system



Aug. 30, 1938. G. MOUNTJOY ET AL AUTOMATIC FREQUENCY CONTROL SYSTEMFiled July 5, 1957 Patented Aug. 30, 1938 UNITED STATE-S PATENT CFFICEAUTOMATIC FREQUENCY CONTROL SYSTEM of Delaware ApplicationJuly 3, 1937,Serial No. 151,794

14 Claims.

Heretofore, automatic frequency control circuits (AFC) have vutilized inthe frequency control network a special control tube functioning toproduce a reactive effect across the oscillator tank circuit. It hasbeen suggested, for the pur- "pose of simplification of circuitarrangement, that the oscillator tube include auxiliary electrodesfunctioning to provide the control tube action. However, such earliercircuits require the use of extra tubes, or specially designedoscillator tubes.

15\There are many situations wherein it is not desirable, and oftenuneconomical, to employ these past arrangements.

. Accordingly it may be stated that it is one of the main objects of ourpresent invention to profvide an automatic frequency control circuit fora superheterodyne receiver, wherein the AFC bias derived from the usualdiscriminator network is applied to one of the existing local oscillatorelectrodes to secure a correction frequency adjust- T-ment in theoscillator tank circuit.

In carrying out such an improved and simplied form of AFC circuit it isimportant, however, to make sure that the controlled oscillatorelectrode, as for example the oscillator grid, is conl nected to a lowimpedance direct current voltage source. If the oscillator electrode isconnected to the output circuit of the well known types of discriminatornetworks, undesirable results are secured since such output circuits areusually `of a high impedance. Again, since the AFC bias should be of asuciently large magnitude to eiiect appreciable control of the tankcircuit, amplification of the I. F. energy prior to rectification by thediscriminator is usually performed. However, where economy of tube usageis a consideration, such amplification can not be secured.

Hence, it may be stated that it is an important object of our inventionto utilize one of the existing. amplier tubes, such as the I. F.amplifier, as a direct current ampliiier for the purpose of amplifyingthe AFC bias derived from the discriminator network; and the ampliedbias being applied from the cathode circuit, which has a low impedance,of the I. F. amplifier to one of the existing electrodes of the localoscillator for the purpose of adjusting the oscillator tank circuitfrequency when the I. F. energy shifts in frequency from the assigned I.F. value.

Another important object of our present inven-J tion'isto' provide, ingeneral, a novel method of (Cl. Z50-20) varying the frequency of anoscillator circuit supplementally of -a main frequency adjustmentdevice; the method involving the utilization of quadrature elements inat least one of the electrode circuits ofthe oscillator for rotating thephase of the current flowing in the electrode circuit with respect tothe alternating voltage applied thereto, thereby to vary the effectivereactance of the oscillator tank circuit.

Still another object of our invention is to improve existing AFCsystems, commonly employed in superheterodyne receivers, by employingthe existing local oscillator electrodes as the frequency controlelectrodes; and the AFC bias being amplified by one of the I. F.amplifiers prior to impression of the AFC bias on one of the localoscillator electrodes.

The novel features which we believe to be characteristick of ourinvention are set forth in particularity in the appended claims; theinvention itself, however, as to both its organization and method ofoperation will best be understood by reference Vto the followingdescription taken in connection with the drawing in which we haveindicated diagrammatically a circuit organization whereby our inventionmay be carried intoeffect.

Referring now to the accompanying drawing, there is shown the circuitdiagram of a lsuperheterodyne receiver embodying generally a signalcollector A; a tunable radio frequency amplifier; a converter; an 1. F.amplifier; a combined second detector-audio amplier; and an AFCdiscriminator. The collector A may be a grounded antenna circuit;' butit may be a tap to a radio distribution` line,l or it can be a loopantenna, .and it can even be the usual signal pick-up device on mobilestructures. The amplifier tube l, any well known type of radio frequencypentode tube, has its signal grid connected to the high alternatingpotential side of the tunable input circuit 2. The input circuitincludes the variable tuning condenser 3 adapted to vary the tuning ofthe amplier over the receiver signal frequency range. The. latter maycomprise a range of 500 to 1500 kc., that is, the broadcast range. .Thecathode of tube l is connected to ground through a bias resistor 4; aradio frequency bypass condenser.

being employed to shunt the resistor 4, and a similar bypass condenserconnecting the low potentialV side of input circuit 2 to ground.-

The input circuit 2 is magnetically coupled to thev collector A; theplate of amplifier tube I being connected to a source of positivepotential. latter-.source is vnot shown, but it is to be understood thata common bleeder circuit can be used for this purpose. The variouselectrodes of the different tubes will be connected to points of properpotential on the bleeder. The cathode side of resistor 4 is connected toa proper positive point on the bleeder to raise the cathode potentialabove ground; the potential of the amplifier cathode with respect toground may be about 7 volts.

The plate circuit of amplifier I is coupled to the signal input circuit5 of the converter (combined local oscillator-first detector) tube 5.The latter may be a pentagrid converter tube of the well known 6A8 type;the tube comprising a cathode l, an oscillator grid electrode 8, anoscillator anode electrode 9, a signal input grid I and an output plateII. The signal grid |0 is electrostatically shielded by positive screenelectrodes; and the plate is connected to a positive potential pointthrough the primary winding I2 of the coupling transformer M. Acondenser I3 resonates the coil I2 to an operating intermediatefrequency (hereinafter referred to as I. F.); the frequency may bechosen from a frequency range of '75 to 450 kc.

The signal input circuit includes the variable tuning condenser I4 whichis adapted to vary the tuning of the circuit over the broadcast range;the high potential side of circuit 5 is connected to signal grid I0,while the low potential side of the circuit is connected to ground by aradio frequency bypass condenser. 'Ihe oscillator anode 9 is connectedto the high alternating potential side of the tunable oscillator tankcircuit comprising coil I5, variable tuning condenser I6 and paddercondenser I 'I. The junction of condensers I6 and I1 is established atground potential; the low potential side of coil I5 being connected to apoint of positive potential through a resistor I8 having a magnitude ofapproximately 10,000 ohms. The oscillator grid 8 is connected to thehigh potential side of feedback coil I9 through a pair of condensers and2| arranged in series.

Condenser 20 may have a value of approximately 10 mmf., and is shuntedby resistor 22 having a magnitude of about 50,000 ohms; condenser 2|having a value of about 100 mmf. The coils I9 and I5 are magneticallycoupled to provide the local oscillations; coil I9 being connected, atits low potential terminal, to the junction of coil I5 and condenserI'I. The cathode 'I of converter tube 6 is connected to ground throughthe bias resistor 23; the cathode being raised above ground byconnecting the cathode side of resistor 23 to a proper positivepotential point of the bleeder. 'I'he converter cathode may beestablished at a potential of approximately 9.5 volts above ground.

The rotors of the tuning condensers 3, I4 and I6 are arranged formechanical unicontrol; the dotted line 24 denotes such controlmechanism. The tank circuit I 5-I'I-I 6 is tunable over a frequencyrange which constantly differs from the signal frequency range by theValue of the I. F. The padder condenser I 'I acts, in a manner wellknown to the art, to maintain the I. F. value constant at all stationsettings of the tuner 24. The local oscillations beat with the signalenergy to produce the I. F. energy. Those skilled in the art are fullyaware of the action of conversion which takes place in tube 6. Briefly,the electron stream thereof is modulated by both the signal andoscillation frequencies; electronic coupling, is employed to produce theI. F. energy in circuit |2-I3.

The coil I2 is coupled to coil 3| in the input circuit of the I. F.amplifier |02; the input circuit is resonated to the operating I. F. Thecathode of tube |02 is connected to ground through a path includingresistors |03 and |04 in series. The resistor |04 is shunted by I. F.bypass condenser 32; resistor |03 may have a magnitude of 400 ohms,While resistor |04 has a value of 500 ohms. The cathode side of resistor|03 is connected to the oscillator grid B by a path including lead 30,resistor 25 and coil 24. The resistor 25 acts as a filter element, whilethe coil 24 functions in a manner to be described at a later point.Condenser 33 is connected between the AFC lead 30 and ground, and actsas an I. F. bypass. The condenser establishes the low potential side ofinput circuit 3 |-3 I at ground potential for alternating currents;switch 35 is an on-off control element for the AFC line. The switch 35has its xed contact connected to the low potential side of coil 3|,while the adjustable element thereof is connected to the junction ofresistors H13-|04.

The plate of the I. F. amplifier |02 is connected to a source ofpositive potential through coil 35; the latter being resonated to theoperating I. F. by the condenser 3l. The coil 35 is magneticallycoupled, as at M1, to the coil 38 of the discriminator input circuit.Coil 30 has its midpoint c'onnected to the plate side of coil 35 by thecondenser 39; the latter may have a magnitude of approximately 100 mmf.The coil 38 is resonated to the operating I. F. or center frequency byits shunt condenser. The discriminator employs a double diode tube,known as a SHS type tube; the anode II of one diede section beingconnected to one side of coil 33, and the anode l2 of the second diodesection being connected to the other side of coil 38. Cathode 4| isconnected by lead I3 to the junction of resistors |03 and |041; whilecathode 42 is connected by lead 44, which includes resistor |00 therein,to the low potential Side of coil 3l.

The resistor I 06 may have a magnitude of approximately one megohm; andthe resistors |00 and IDI, connected in series between cathodes 0I and02', may have a Value of approximately one megohm. The junction ofresistors |00 and IBI is connected by lead i5 to the midpoint of coil38; the I. F. bypass condenser i0 being connected in shunt across theresistors |00-IOI- The point 09 of resistor IIII, to which lead i4 isconnected, has a variable direct current potential with respect to point99' when the I. F. energy shifts in frequency from the assigned I. F.value. The potential is amplified by the tube |02, and appears inamplified form across resistors I {i3-|00; the AFC lead 30 applies theamplified direct current potential to the oscillator grid 8 foroscillator frequency adjustment. The I. F. energy is demodulated forsecuring audio and AVC voltages by tube 50; the latter may be adiode-triode tube of the 6Q'7G type. 'Ihe cathode of the tube isconnected to ground through a path including resistors |07 and |08 inseries; the resistor I 0l may have a magnitude of approximately 200ohms. The cathode side of resistor I0'I is connected to a properpositive potential point on the bleeder; the cathode is at about 4.5volts above ground.

The diode anode 5| of tube 50 is connected to the cathode of the latterthrough a path including coil 52, link coupling coil 53, resistor 54 andresistor 55. The coil 53 is of but a few turns, and is magneticallycoupled to coil 33 at the midpoint of the latter. Resistors 54-55 areshunted by the I. F. bypass condenser 55, and the resistors are of about50,000 ohms and 250,000 ohms respectively.; The condenser 'I resonatesthe coils 52=53 to the assigned I. F. value; AVC bias is taken off fromthe junctionfof resistors 54 and 55.1; The AVC lead 60, including theresistor-ca- .'.pacity filter network 6i, is connected from the junctionof resistors 54 and 55 to the low potentialsides of input circuits 2 and5. The AVC connections'to the signal grid circuits of tubes I and 6.include appropriatefilter resistors 6I. The audio component ofthedetected I. F. energy is impressed on the control grid of the tube 56byV April 3, 1936, Patent No. 2,120,974, dated June 2.1,

1938 by D. E. Foster. For this reason it is not believed necessary todescribe Athe operation of thesenetworks in detail; a generalexplanation will now be given.

The theoretical basis for the production of the AFC voltage acrossresistors Ili-l 6I is explained in the following manner. The potentialsat either end of coil 38, with respect to its midpoint, are 180 degreesout of phase. Hence, if the midpoint is Q connected to the highpotential side of coil 36, one

potential is realized which maximizes above the resonant frequency ofthe I. F. value, and a second potential is realized which maximizesbelow this value. If these two potentials are applied to a 404;.pair ofrectiers, such as the diodes of tube 40,

and the resulting direct current voltages are added in opposition, thesum will be equal to zero. Inthe -type of discriminator network shown inthe drawing, the primary and secondary coils 36 and 38 are solconnectedthat two Vector sum potentials `of the primary and secondary voltagesmaybe realized.v When the I. F. energy departs in frequency value fromthe assigned operating I. F.',then there is developed across resistorsIMI and. `IIlI ,a direct current voltage. Since point 99 is connected bylead i3-to the junction of resistors |03 and IM, then the point 99 willvary in potential in magnitude and polarity depending upon the-amountyand-.direction of shift of the I. F.

, energy.

Of course, any other type` of discriminator for producing a directvoltage from the I. F. energy, when; the latter shifts in frequency fromthe assigned I. F. value, may be utilized. Such other networks are wellknown to those skilled in the art. In general, it may be stated thatthere may be employed a source of direct current voltage, which voltageis derived from the I. F. energy when the latter departs fromtheassigned I. F.

value; and the direct current voltage being produced in a sense tovprovide `correction bias for the'local oscillator network. The separatediode rectifier; arranged in tube 50, is employed so as tosecureadequate selectivity even though but one .stage of I. F.amplification is employed. The diodeof 4tube 56 has its tuned inputcircuit coupled to the coil 38, and hence it will rbe seen that theinput circuit4 of the second detector diode is really a tertiary tunedcircuit coupled to the ,secondaryA circuitincluding coil 38. Thecoupling between coils 38 and 53 is arranged so that coil 53 is coupledonly to coil 38, and not to coil 36. This is done, as schematicallyshown in the drawing, by using a few coupling turns 53 close to coil 38.These turns are shown at the center of coil 38, and this arrangement isphysically followed in order to keep the capacity coupling of coil 53symmetrical with respect to both sides of coil 38.V Inthis wayin spite.of the use of but a single I. F. amplifier, the selectivity precedingthe audio demodulator is satisfactory.

The AVC arrangement shown acts, of course, to reduce the gain ofamplifier I, as well as to reduce the conversion gain of converter 6, asthe signal carrier amplitude at the second detector input circuitincreases. In this way the carrier amplitude at the audio demodulatorinput circuit is maintained substantially uniform in spite of variationsin signal amplitude at the signal collector. Of course, the carrieramplitude at the discriminator input circuit also remains substantiallyuniform in spite of signal amplitude variations at the signal collector.

The AFC bias is applied by lead 30 to the oscillator grid 8. This causesa change in the` frequency, at any setting of the tuner 24, of theoscillator tank circuit. rIhe oscillator strength is fairly constantwith changes in bias. The conversion conductance increases with anincrease of negative oscillator Voltage. This effect is in partcompensated for by the fact that an increase in negative oscillatorvoltagel is obtained by a decrease in the I. F. amplifier tube current,with an attending decrease in the mutual conductance of the I. F.amplifier |02. There will now be described the mechanism by which theoscillator tank circuit is shifted in frequency as soon as AFC bias isproduced. It is to be clearly understood that our explanation involves,in part, theoretical aspects, and we are not restricted to such aspects.The frequency changes occur as described. n

The oscillator produces an oscillatory voltage across the tank circuitI5, I6, I1, which is transferred to coil I9 by mutual inductance. In anoscillator the grid draws current so that there is a finite gridimpedance, predominantly resistive, if the grid-cathode static capacitycan be neglected. For the purposes of explaining the action thiscapacity will be neglected and its effect shown later. The voltage inthe grid coil I9 causes a current to iiow through resistor 22, capacity26 and the input grid resistance of grid 8 of tube 6. Capacity 2I is ablocking capacity and does not iniiuence control action. Capacity 20 andresistor 22 and the grid resistance rg of the tube rotate the phase ofthe current relative to the voltage in that circuit. The current flowingthrough rg therefore has an in-phase and a quadrature component. Thein-phase component serves to maintain oscillation and the quadraturecomponent to vary oscillation frequency. These two components of currentowing through 'rg produce the grid voltage at oscillator frequencywhich, by virtue of the mutual conductance of the tube 6, produces analternating current in the plate circuit thereof. The direction of themutual inductance between coils I5 and I9 necessary to maintainoscillation is such that this alternating plate current produces aneffect equivalent=to a negative resistance and a negative capacity inshunt to the tank circuit.

The negative resistance component is responsible for the oscillation andthe negative capacity.Y component varies the frequency to some Value.

other than that due to the constants of the tank circuit alone. Now ifthe direct current is applied to grid 8 of tube through inductance 24"and resistance 25 the effect on tube 6 is to vary the mutual conductanceand the grid resistance rg. If the direct current bias on grid ES` ismade more positive the mutual conductance increases but the gridresistance decreases. The effect of these two factors being oppositetends to maintain the oscillatory voltage substantially constant unlessthe bias on grid 3 is made so negative that cut-off is approached orexceeded, in which case the oscillator ceases to function. If directcurrent bias on grid 3 is made positive, the effect on the negativecapacity due to the quadrature component is chiefly that of thevariation of rg. In this case rg decreases and the negative capacity inshunt to the tank circuit increases and the oscillatory frequencyincreases. An increase in negative capacity in shunt to a tuned circuithas the same effect on frequency as a decrease in a positive inductancein shunt to the tuned circuit. However, a negative capacity has anirnpedance which decreases with increasing frequency whereas a positiveinductance has an irnpedance which increases with increase in frequency. The frequency shift produced by a given change in negativecapacity is thus greater at the high frequency end of the tuningspectrum Where the tank circuit capacity is low, whereas the effect of agiven change in positive inductance is a substantially constantpercentage of 'the frequency over the tuning spectrum, since the tankcircuit inductance is a constant. It has been shown that making thedirect current potential of grid 8 more positive results in an increasein oscillatory frequency. If the direct current potential of grid 8 ismade more negative the frequency decreases. It has been shown, also,that the effect of capacity 2li is to produce a negative capacity inshunt to the tuned circuit. If capacity Z is replaced by an inductancethe effect would be as if a negative inductance were shunted across thetuned circuit, in which case the amount of shift would be more constantover the tuning spectrum.

However, there are advantages in using a capacity in the grid circuit,namely it is possible to secure a capacity without inductive effect, butall inductances have associated therewith some distributed capacitywhich may disturb the desired phase relations at some tuning frequency.Furthermore, there is less change in the amount of frequency shift whena capacity is used than the foregoing description of operation wouldshow, because of the effect of the static capacity of grid 8 to cathodeTl of tube 6. This capacity shunts rg and exhibits a lower reactan'ce athigh frequencies than at low frequencies and therefore tends to decreasethe variation in frequency shift obtained with variation of frequencyover the tuning spectrum.

Inductance 2t acts as a choke coil so that resistor 25 may be made lowin value without decreasing the oscillation amplitude. inductance 2li ispreferably made of such value as to be resonant with the internalcapacity between grid 8 and cathode 'F of tube at or below the lowesttuned frequency, so that the effect of this internal capacity ispartially cancelled at the low frequency end of the spectrum to enhancethe frequency shift at those tuned frequencies. A value of inductance213 of 2.4 millihenries has been found suitable when tube 6 is of the6A7 or GASG type.

Resistor Z5 improves the ease of starting oscillation by giving a smallamount of self bias to grid 8 of tube 6. It is preferably made as smallas consistent with this requirement in order that any variations indirect grid current of grid 8, due to changes in oscillator `amplitudeover the tuning spectrum, will not produce large changes in bias on thatgrid; as it is required for purposes of frequency shift, that changes inbias of grid 8 be due predominantly to changes of discrimi` natorpotential caused by intermediate frequency shift. In an experimentalreceiver a value of 2500 ohms has been found suitable for resistor 25.

In the operation of this circuit it has been found that making the biasof grid 8 of tube G more negative resulted in increased conversionefficiency of that tube, that is, increased intermediate frequencyoutput with constant radio frequency input signal intensity. This iscontrary to what might commonly be expected but can be explained by theobservation noted above that rg and mutual conductance of grid 8 vary inopposite directions with bias thereon, tending to maintain substantiallyuniform oscillatory voltage across rg. But if the oscillatory voltage isconstant and fg increases with increasing negative bias on grid 8, thecurrent flowing to grid 8 must decrease since the oscillatory voltage ongrid 8 is the product of rg and the oscillatory current. The decreasedcurrent flow to grid 8 results in increased current flow to the otherelectrodes and consequently increased conversion efficiency. As has beenseen, the bias on grid 8 has a determining effect on the mutualconductance between grid 8 and electrode yhowever, electrode 9 haslittle effect on the flow of current to anode il of tube but the flow ofcurrent to grid E does influence the output from anode Il so that a morenegative bias on grid 8 results in increased conversion efficiency andvice versa.

It is desired to point out at this time that the padder condenser ilneed not be located in the path common to both grid and plate circuitsof the oscillator. For example, it can be disposed in series relationwith the variable condenser it; and in that case the common junction ofcoils l5 and l would be by-passed to ground through a radio frequencyby-pass condenser. Of course, it is not essential to the operation ofthis circuit to employ a padder condenser, and the latter may bedispensed with if desired. Moreover, it is to be understood that theoscillator section may include a tube independent of the mixer section.Of course, the quadrature elements 2%-22 are illustrative in nature;they may be replaced by combinations of elements chosen to produce theoscillator frequency shift which is desired. Such combinations ofelements may in general include inductive, capacitative and resistiveelements. These elements may be used singly in proper cases.

As has been previously explained there is developed across thediscriminator output resistors l0@ and l'l a direct current voltagewhich varies in magnitude and polarity in dependence upon the amount offrequency departure of the I. F. energy from the operating I. F. as wellas upon the direction of departure. 'I'here will now be given anexplanation of how the Variable direct current voltage is utilized toproduce a shiftsofi" ingresistor of tube |02. Resistor |04 is a voltageamplifying resistor in the cathode circuit of tube E02, but serves nofunction in biasing tube |02,

While the I.F. tube was chosen for this function' inthis specific case,obviously a separate tube might provide the same action, and obviouslyan R. F. amplifier tube or other tube in the rec eiver might be chosento provide the same results.

lDiscrirninator volts at center or correct frequency are zero inmagnitude. The bias on tube |02 is then only the self bias and the platecurrent of tube |02 is the result of the operating potential and tubecharacteristics. This vplate current provides a voltage drop acrossresistances |03 and |04 in series, which voltage drop is applied to thegrid 8 of tube 6 through resistance and coil 24.

The effective xed bias on grid 8` of tube 6 is the voltage acrossresistors ||03 and |04 plus the voltage across resistor 23 in thecathode circuit of tube 6. The voltage across resistor 23 is produced bythe ow of current from cathode 1 and resistor |05 connected to a highpotential source B plus. Variations in the voltage across resistors |03and |04 will vary the bias on grid 8. When an input signal is applied toantenna A of lower frequency than necessary to produce by ,beating withthe oscillator frequency an I. F.

of correct frequency or predetermined frequency, an I. F. signal isproduced which is higher than the predetermined frequency.

" Signals of this higher frequency are passed through the discriminatorIcircuit and a discriminator voltage is developed across resistors |00and |01, which voltage is negative as automatically applied to thecontrol grid of tube |02. A decrease in cathode current in tube |02results in a decrease in voltage across resistors |03 and |00 thuschanging the bias on grid 8 of tube 6 and producing a more negative biason grid 8. This more negative bias shifts the frequency of oscillationas previously explained and causes a lower frequency to be produced.This lower oscillator frequency beating with the input signal produces alowering in the I. F. signal frequency and tends to correct for theassumed condition of I. F. signals higher than the desired I. F.

With an input signal to the antenna A is higher in frequency than thatnecessary to produce the correct I. F. by beating with the oscillatorfrequency, the reversal of the above biasing process takes place and theI. F. frequency is corrected. v The control characteristics of theoscillatorcontrol circuit were experimentally measured o n theexperimental receiver from which the schematic diagram was derived. Theadvisability of van initialV biasing point of about 3.5 volts negativeon grid 8 was determined. In consequence values of components werechosen to provide the following D. C. voltages:

Across R23, 9.5 volts Across R103, 2.7 volts Across R104, 3.3 voltsAcross R4, 7 volts Across R107 and 10s, 5 volts These voltages result inthe following biases on the several tubes to be described. Grid 8 intube 0 is biased 3.5 volts negatively. The control grid of the radiofrequency amplifier tube l is biased 2 volts negatively. On grid I0 oftube 6 the bias is 4.5 volts negative. On the control grid of tube |02the bias is 2.7 volts negative. It is to be understood, of course, thatthe foregoing values are merely'illustrative in'nature, and are in noway restrictive.

It is -emphasized that the AFC line 30 is connected to a low impedancesource of direct current voltage. The magnitude of shift of theoscillator frequency is dependent upon the change in bias existingbetween grid 8 and cathode of tube E. If a high impedance source of D.C. bias were used the condition of grid current characteristic ofoscillator circuits wouldprovide a voltage drop through th-e internalimpedanceof the bias source which drop would subtract from the Voltageof the bias source in the form of a loss, and would not provide a biaschange effective between grid 8 and cathode. Thus the resistors |03 and|04 provide the double function of permitting the development of thebias voltage, and by virtue of their low impedance minimize the loss inbias voltage actually transferred to the control grid 8.

As explained previously 3.5 volts may be employed forthe oscillator biasat the operating I. F. value. The voltage drop across the I.v F.amplifier cathode impedance will change with 01T- frequency signal plusor minus 3 volts. Experimentally, it was determined that the totaleffective shift was approximately 14 kc. at a setting of the tuner 24 of1400 kc.; 11 kc. at 1000 kc. setting; and '7 kc. at 600 kc. setting.,Since frequency drift and inaccuracies of mechanical tuning devices arelarger at the high frequency end of the spectrum, the increased controlshift at 1400 kc. is advantageous. v

It will now be seen that we have provided, by virtue of our presentinvention, a superheterodyne receiver Which'employs a single stage oftunable radio frequency amplification followed by a Pentagrid converterstage and a stage of I. F. amplification; a discriminator network beingemployed to furnish the AFC bias, and the latter being impresseduponanelectrode of the oscillator section of the converter. A satisfactoryAFC circuit is provided by means of the present invention, an existingtube of the receiver being employed to amplify the AFC bias. In thepresent system of automatic frequency control it is not necessary toutilize a separate frequency control tube; on the contrary, theelectrodes of the oscillator are employed for the frequency controlfunction. The quadrature elementsy 20 and 22, and the choke coil 24',are the only additions to the local oscillator circuit. In general,then, our present invention makes it possible to employ AFC in asuperheterodyne receiver of compact construction wherein it is arequirement that the receiver employ a minimum of tubes.

While we have indicated and described a system vfor carrying ourinvention into effect, it will be apparent to one skilled in the artthat our invention is by no means limited to the particular organizationshown and described, but that many modifications may be made withoutdeparting from the scope of our invention, as set forth in the appendedclaims.

What we claim is:

1. In a superheterodyne receiver of the type employing a localoscillator circuit provided with a tank network having means for tuningit over a desired wide frequency range, said circuit including a tubeprovided with a cathode, anode and control grid electrode, said anodeand grid elec-` trode being reactively coupled, and said tank networkbeing connected between said anode and cathode, means for varying thedirect current potential of the oscillator grid electrode, resistanceand reactance elements connected in the oscillator grid circuit, andsaid elements being so chosen as to Vary the frequency of the oscillatortank network in response to variations in said direct current potential.

2. In a receiver as defined in claim 1, said elements being arranged inshunt relation to each other, and the shunted elements being connectedin series in the oscillator grid circuit.

3. In a receiver as defined in Claim l, an intermediate frequencytransmission network, and said varying means comprising a networkresponsive to shifts in the frequency value of the intermediatefrequency energy.

4. In a receiver as defined in claim 1, an intermediate frequencyamplifier, a discriminator network coupled to said amplifier andproviding said varying means, a connection between the discriminatoroutput and the input circuit of the amplifier, and an additionalconnection between the output circuit of said amplier and saidoscillator grid.

5.V In a receiver as defined in claim 1, said varying means including aninductance connected to said oscillator grid electrode, and saidinductance having a magnitude such as to resonate the oscillatorgrid-cathode capacity to a frequency adjacent the low frequency end ofsaid frequency range.

6. In combination with an electron discharge tube of the type includingat least a cathode, an anode and a control electrode, a resonant tankcircuit connected between the cathode and at least one of the other twoelectrodes of the tube, means for reactively coupling the secondelectrode to said tank circuit, means connected to said second of thetube electrodes for rotating the phase of the current flowing in thecircuit thereof with respect to the alternating voltage applied theretothereby Varying the effective reactance of saidtank circuit, and meansfor varying the magnitude of the current flowing into said tank circuitto control the effectiveness of said phase rotating means.

7. In an oscillator network of the type including a tube provided with acathode electrode, an anode electrode and a control electrode, meansproviding a resonant tank circuit between the cathode and at least oneof the other two electrodes, impedance means coupling said twoelectrodes in such a manner as to provide an oscillatory current flowthrough said tank circuit, means connected to the second of said tubeelectrodes for rotating the phase of the current iiowing in the circuitthereof with respect to the alternating voltage applied thereto therebyto vary the effective reactance of said tank circuit, and means forvarying the magnitude of the current flowing in to said tank circuit tocontrol the effectiveness of said phase rotating means.

8. In an oscillator as defined in claim '7, said varying meanscomprising a source of variable direct current potential.

9. In an oscillator as defined in claim 6, said phase rotating meanscomprising at least a reactance.

10. In a superheterodyne receiver, a local oscillator having a resonanttank circuit, said oscillator being of the type including at least acathode,

a control grid and an anode, said tank circuit being connected betweenthe anode and cathode, said grid being reactively coupled to the tankcircuit, a network electrically associate-d with the oscillator forproducing energy of an intermediate frequency, means for deriving adirect current voltage from the intermediate frequency energy inresponse to a frequency departure of the latter from a predeterminedfrequency value, and means for applying said direct current voltage tosaid oscillator control grid thereby to adjust the frequency of the tankcircuit in a sense to maintain said predetermined frequency value.

l1. In a superheterodyne receiver, a local oscillator having a resonanttank circuit, said oscillator being of the type including at least acathode, a control grid and an anode, said tank circuit being connectedbetween the anode and cathode, said-grid being reactively coupled to thetank circuit, a network electrically associated with the oscillator forproducing energy of an intermediate frequency, an intermediate frequencyamplifier for amplifying said intermediate frequency energy, means forderiving a direct current voltage from the amplified intermediatefrequency energy in response to a frequency departure of the latter froma predetermined frequency value, means for impressing said directcurrent voltage upon said amplifier for` amplification of the voltage,and means for applying said amplified direct current voltage to saidoscillator control` grid thereby to adjust the frequency of the tankcircuit in a sense to maintain said predetermined frequency value.

12. In a receiver as defined in claim l0, means connected to theoscillator control grid for rotating the phase of the current flowing inthe control grid circuit with respect to the alternating voltage appliedthereto.

13. In a superheterodyne receiver of the type employing a first detectornetwork having a tunable signal input circuit, a local oscillatornetwork having a tank circuit including means for tuning it over anoscillation frequency range, said oscillator network including a tubehaving a cathode, anode and grid electrode, said tank circuit beingconnected between the anode and cathode, said grid being reactivelycoupled to said tank circuit, an intermediate frequency networkincluding at least one amplifier, and a discriminator circuit forderiving a direct current voltage from the amplifier output energy whenthe latter departs in frequency from a predetermined intermediatefrequency Value, the improvement which comprises means electricallyconnected to the oscillator grid electrode for producing a change in theeffective reactance of said oscillator tank circuit, Yand additionalmeans for impressing said discriminator direct current voltage upon theoscillator grid electrode in a sense to control said effective reactancechange in that direction which will maintain said predeterminedintermediate frequency value.

14. In a receiver as defined in claim 13, said additional meansincluding said one amplifier functioning as a direct current amplifier.

GARRARD MOUN TJ OY. DUDLEY E. FOSTER.

